The present invention relates to superconducting-coil current leads which are used to connect a power supply placed in a room-temperature environment to a superconducting coil placed in an ultralow-temperature environment.
A strong magnetic field utilized for the confinement of plasma in a reactor, such as a nuclear fusion reactor, is generated by means of a superconducting coil. A superconducting coil used for such a purpose is kept at an ultralow temperature of 4K or so, but a power supply for exciting the superconducting coil is kept at room temperature. Therefore, a current lead, which is part of an electric circuit including the power supply and the superconducting coil, includes portions kept at room temperature and portions kept at ultralow temperature. In the current lead, the heat conduction arises from the temperature difference and Joule heat is generated by current flow, and heat travels from the room-temperature portions to the ultralow-temperature portions. The amount of heat traveling from the room-temperature portions to the ultralow-temperature portions is larger than a half of the total amount of heat entering the large-sized superconducting coil system. To ensure a stable and economic operation of the superconducting coil, it is preferable that the heat conduction from the room-temperature portions to the ultralow-temperature portions be suppressed to a possible degree.
A gas-cooled current lead, such as that shown in FIG. 1, is employed to reduce the amount of heat that enters the system through the current lead. With respect to the current lead, the mathematical product between the heat conductivity and the electrical resistance should be as small as possible. Usually, therefore, current leads are formed of normal conductors, i.e., metals such as Cu and Al. As shown in FIG. 1, a superconducting coil covered with a conduit 3 is immersed in the liquid helium 2 contained in a cryostat 1. A large number of superconducting strands 4 are led out of the conduit 3 and connected to the respective current lead strands 5. The current lead strands 5 are housed inside a current lead tube 6 and led out of the cryostat 1. The use of a large number of current lead strands is useful in increasing the ratio of the surface area to the cross sectional area.
Referring to FIG. 1, the liquid helium 2 gasifies due to the heat that enters the system through the current lead strands 5. The resultant cold helium gas passes through the current lead tube 6 and exchanges heat with reference to the current lead strands. Then, the helium gas flows out from the upper portion of the current lead tube 6. Since, in this manner, the current lead strands 5 are cooled by the cold helium gas, the heat conduction to a lower temperature region is suppressed.
However, even if the gas-cooled current lead mentioned above is employed in a large-sized heavy-current superconducting coil system, the amount of heat that enters the system from the current lead is inevitably large. Therefore, in light of the manner in which electric power is utilized in practice, the use of the gas-cooled current lead necessitates a high expense for operation or maintenance and is not desirable in the economical aspects. Hence, the amount of heat entering the system has to be reduced more efficiently.
Under these circumstances, more and more researches are recently made to provide a current lead wherein a normal conductor is employed in a room-temperature region and a high-temperature superconductor (HTS) is employed in an ultralow-temperature region. An example of such a current lead is shown in FIG. 2. Referring to this FIGURE, a power supply 100 placed in a room-temperature environment and a superconducting coil 200 placed in an ultralow-temperature environment are connected together by means of a current lead 11, which is obtained by joining a normal conductor 12 and a high-temperature superconductor 13 together. A high-temperature superconductor recently developed does not have an electric resistance even at the temperature of a liquid nitrogen (77K) or thereabouts, as long as it is placed in a low magnetic field. This being so, the high-temperature superconductor allows conduction of a large amount of current, and yet it does not generate heat owing to superconduction. In addition, where it is formed of a Bi-based material (Bi-2223, Bi-2212) or a Y-based material, the heat conductivity which it has at a temperature of 100K to 10K is about 1/1,000 of that of copper. Due to these characteristics, the use of the high-temperature superconductor is effective in suppressing the heat which may enter the system by way of the current lead 11.
The inventor of the present invention previously proposed a current lead that utilized a Peltier effect (an example of such a current lead is shown in FIG. 3), and named it a Peltier current lead. This Peltier current lead is made up of a first current lead 21a and a second current lead 21b, the former being obtained by joining an N-type thermoelectric semiconductor 22a, a normal conductor 23 and a high-temperature superconductor 24 together, and the latter being obtained by joining a P-type thermoelectric semiconductor 22b, a normal conductor 23 and a high-temperature superconductor 24 together. By means of the first and second current leads 2la and 21b, the Peltier current lead connects a power supply 100 located in a room-temperature environment and a superconducting coil 200 located in an ultralow-temperature environment. The N- and P-type thermoelectric semiconductors 22a and 22b are formed of a BiTe-based material or a BiTeSb-based material. In the current circuit formed by the Peltier current lead, a current from the power supply 100 flows first through the first current lead 21a, then through the superconducting coil 200, then through the second current lead 21b, and then returns to the power supply 100.
When a current is supplied to the N- and P-type thermoelectric semiconductors 22a and 22b of the current leads 21a and 21b, as indicated by the arrows shown in FIG. 3, the thermoelectric semiconductors 22a and 22b exhibit the Peltier effect and thus function as a heat pump. Thus, heat is conveyed from the low-temperature region to the room-temperature region. In the case where the thermoelectric semiconductors 22a and 22b are formed of a BiTe-based material or a BiTeSb-based material, they can cool an object to as low as 200K or thereabouts in the state where there is no heat load. As a result, those portions of the current leads 21a and 21b which are located in the room-temperature environment are cooled, and heat is not transmitted to the ultralow-temperature portions of the system.
The high-temperature superconductor 24 is used at a temperature lower than that of liquid nitrogen. In practice, however, it cannot be cooled to this low temperature if the thermoelectric semiconductors are formed of a BiTe-based or BiTeSb-based material. This is why the normal conductors 23 are inserted between the thermoelectric semiconductors 22a, 22b and the high-temperature superconductors 24. At room temperature or thereabouts, the thermoelectric semiconductors formed of the BiTe-based or BiTeSb-based material has a heat conductivity which is about 1/200 of that of copper. Hence, heat is not transmitted to the ultralow-temperature region even when no current is supplied.
Even when the current leads shown in FIGS. 2 and 3 are employed, the amount of heat transmitted to the ultralow-temperature region through the normal conductors cannot be neglected. It is therefore desired that the heat transmitted to the ultralow-temperature region by way of the current leads of the superconducting coil be reduced further.